This invention relates to a plating bath for electrodeposition of aluminum, having a high conductivity and a high current efficiency, and more particularly a molten salt bath, comprising an aluminum halide and a 1-alkyl- or 1,3-dialkylimidazolium halide, and a plating process making use of the bath.
Processes for electrodeposition of aluminum, using a plating bath stable to oxygen or water and capable of being carried out at a relatively low temperature, include a process carried out using a mixed molten salt bath comprising an aluminum halide and a quaternary ammonium salt. A process included in this process and capable of assuring safe operation is a process employing a bath in which an N-alkylpyridinium halide is used as the quaternary ammonium salt. For example, U.S. Pat. Nos. 2,446,331, 2,446,349, and 2,446,350 disclose processes in which aluminum chloride and N-ethylpyridinium chloride are used as basic components, and U.S. Pat. No. 4,747,916 discloses a process in which an aluminum halide and an N-butylpridinium halide are used as basic components. The former three processes, however, may cause color changes of coatings when the plating is carried out at a high current density of not less than 10 A/dm.sup.2, and hence is not suited to continuous plating for mass-producing those which have uniform appearance. On the other hand, the last process disclosed in U.S. Pat. No. 4,747,916 may not cause any color changes of coatings even when the current density is raised up to 30 A/dm.sup.2, and is suited to the continuous plating, but, if the plating is carried out at a high current density of more than 30 A/dm.sup.2, it has sometimes occurred that gray burnt deposits are generated on coatings, resulting in a lowering of commercial value of the products. Studies made by the present inventors revealed that this is ascribable to the reduction of cations. More specifically, the aluminum halide and N-alkylpyridinium halide are dissociated into Al complex ions such as AlX.sub.4.sup.- or Al.sub.2 X.sub.7.sup.- and N-alkylpyridinium cations, where the latter cations are reduced in the vicinity of -2 V with respect to the electrodeposition potential of Al when observed on the cathode polarization curve, and the reduction reaction of cations takes place when a voltage is excessively applied in carrying out the Al electrodeposition, thus resulting in the generation of burnt deposits. For this reason, current efficiency is also lowered in the instance where the plating is carried out at a high current density.